Evaluation of Pleural Fluid Cytology for the Diagnosis of Malignant Pleural Effusion: A Retrospective Cohort Study

Abstract BackgroundAs median survival time for patients with malignant pleural effusions (MPE) is nine months, rapid and accurate diagnosis is important. Cytological examination of pleural fluid has a good specificity but imperfect sensitivity. Published estimates of sensitivity vary substantially and predictors of false negative cytology are not well established. We aim to establish a local estimate of pleural fluid cytology sensitivity and identify risk factors for false negative cytology.MethodsWe conducted a retrospective cohort study of patients who had cytology testing of pleural fluid at Christchurch Hospital, New Zealand 21 July 2017 to 31 October 2019. Data on patient demographic, clinical and pleural fluid characteristics were collected. MPE was defined by positive pleural fluid cytology, tissue histology, or multi-disciplinary meeting consensus. We estimated sensitivity of the first pleural cytology assessment. We performed multivariate logistic regression to ascertain patient groups at greatest risk of false negative results. ResultsOf 156 patients with confirmed malignant pleural effusion included in our study, the initial pleural fluid cytology was diagnostic in 117, providing a sensitivity (95% confidence interval) of 75.0% (67.4-81.6%). The sensitivity was 79.0% (66.8-88.3%) for lung cancer, 91.3% (72.0-98.9%) for breast cancer and 33.3% (95% CI 11.8-61.6%) for mesothelioma. Cloudy appearance of pleural fluid (OR 0.17, 95% CI 0.04-0.84), but not pH, lactate dehydrogenase or polynuclear cell ratio, reduced the odds of false negative pleural cytology.ConclusionPleural fluid cytology was sensitive in diagnosing MPE due to primary lung and breast cancer, however it had low sensitivity in mesothelioma. Clinicians should be particularly alert to the high likelihood of false negative results when suspecting mesothelioma and consider early use of biopsy.